Ultrasonic transmit and receive system

ABSTRACT

A transmit beamformer includes multiple transducers, each responsive to a respective transmit waveform to produce a respective transducer waveform. A transmit waveform generator generates the transmit waveforms, and the transmit waveforms each include multiple frequency components. Progressively higher frequency components of the transmit waveforms are timed to cause corresponding progressively higher frequency components of the transducer waveforms to focus along a line at progressively shorter ranges. In this way, a frequency dependent line focus is achieved.

This application is a continuation of application No. 08/926,270, filedSep. 5, 1997, now U.S. Pat. No. 5,933,389, which is a divisional ofapplication 08/771,345, now U.S. Pat. No. 5,696,737, issued Dec. 9,1997, which is a divisional of application No. 08/397,833, filed Mar. 2,1995, now U.S. Pat. No. 5,608,690, issued Mar. 4, 1997.

BACKGROUND OF THE INVENTION

This invention relates to beamformers, and in particular to a transmitbeamformer that provides improved focusing.

Ultrasonic imaging is widely used in many settings, including medicalapplications. A typical ultrasonic imaging system includes an array oftransducers, a transmit beamformer, and a receive beamformer. Thetransmit beamformer supplies transmit waveforms (which may be voltagewaveforms) to the transducers, which in turn produce respectiveultrasonic transducer waveforms (which are pressure waveforms). In aphased array system, the transmit waveforms are delayed in time to causethe ultrasonic waveforms to interfere coherently in a selected region infront of the transducers.

Structures in front of the transducers scatter ultrasonic energy back tothe transducers, which generate associated receive waveforms (which maybe voltage waveforms). These receive waveforms are delayed for selectedtimes that are specific for each transducer such that ultrasonic energyscattered from a selected region adds coherently, while ultrasonicenergy from other regions does not.

It is well recognized that the absorption characteristics of the bodybeing imaged can have a significant impact on the operation of anultrasonic imaging system. For example, the ultrasonic absorptioncoefficient of living tissue increases with frequency, and lowerfrequencies are therefore preferred for imaging at greater depths.Higher frequencies provide improved resolution in the range dimensionthan lower frequencies, and higher frequencies are preferred for imagingat shallower depths.

Pittaro U.S. Pat. No. 5,113,706 discloses an ultrasonic imaging systemthat divides the body being imaged into several zones, and uses aseparate burst of ultrasonic energy at a separate frequency and powerlevel for each zone. In this system, transmit focus and power arestepped over the entire multi-zone focal range of interest, withsuccessive bursts that increase in focal depth, decrease in frequency,and increase in power.

One disadvantage of the system disclosed in the Pittaro patent is thatmultiple bursts are fired for each transducer steering position. Suchmultiple bursts can increase the time needed to complete an entireimage. The Pittaro patent makes a brief suggestion at column 13, lines35-39 that frequency multiplexing can be used so that the multiplewavefronts for a given steering position can be concurrent rather thansuccessive, but no further details are given.

SUMMARY OF THE INVENTION

This invention is directed to an improved transmit beamformer thatreduces or eliminates the need for multiple bursts at a given transducersteering position, and thereby increases the rate at which an image canbe generated while maintaining multiple focal points.

According to this invention, a transmit beamformer generates transmitwaveforms for an array of transducers, which respond by producingassociated transducer waveforms. Each of at least some of the transmitwaveforms comprises at least first and second frequency components whichare included in a single burst of energy. The first frequency componentsare timed to cause corresponding first frequency components of thetransducer waveforms to focus at a first, greater depth, and the secondfrequency components are timed to cause corresponding second frequencycomponents of the transducer waveforms to focus at a second, shallowerdepth.

In the preferred embodiments discussed below, each transmit waveformincludes more than two frequency components, and progressively higherfrequency components of the transmit waveforms are timed to causecorresponding progressively lower frequency components of the transducerwaveforms to focus at progressively greater depths.

As used herein, the term "frequency component" is meant to beinterpreted broadly so as to encompass frequency components having anysuitable bandwidth. Where frequency components have a finite bandwidth,they may be spaced such that adjacent components fill the bandwidth, andare therefore substantially continuous.

These embodiments allow high image-frame rates, since a single set oftransmit waveforms is used to inject energy into short, intermediate andlong range parts of the body being imaged. Furthermore, since thefocused energy is distributed along a line, more energy may be injectedinto the body before power limits such as those imposed by governmentalregulatory agencies are exceeded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an ultrasonic imaging system thatincorporates a presently preferred embodiment of this invention.

FIG. 2 is a graph of a time domain function h(t).

FIGS. 3 and 4 are graphs of the amplitude and phase, respectively, of afrequency domain function H(f), the Fourier transform of h(t).

FIGS. 5 and 6 are graphs of the amplitude and phase of a time shiftedfrequency domain function H'_(i) (f).

FIG. 7 is a graph showing the time development of selected transmitwaveforms.

FIG. 8 is a graph corresponding to the waveforms of FIG. 7 filteredthrough a 3 MHz bandpass filter.

FIG. 9 is a graph of the waveforms of FIG. 7 filtered through a 7 MHzbandpass filter.

FIGS. 10 and 11 are graphs showing the high and low frequency wavefrontsin two alternate sets of transmit waveforms.

FIG. 12 is a block diagram showing a first preferred embodiment of thetransmit beamformer 12.

FIG. 13 is a block diagram showing a second preferred embodiment of thetransmit beamformer 12.

FIG. 14 is a block diagram showing a third preferred embodiment of thetransmit beamformer 12.

FIG. 15 is a block diagram showing a fourth preferred embodiment of thetransmit beamformer 12.

FIG. 16 is a graph of unfiltered and filtered spectra (3, 5, 7 MHz).

FIG. 17 is a graph showing focal distance versus transducer number for3, 7 and 10 MHz components.

FIG. 18 is a graph of spectra of transmit waveforms for the centerthrough end transducers within the frequency range of 0-10 MHz.

FIG. 19 is a contour plot of the spectra of FIG. 18.

FIG. 20 is a graph showing transmit waveforms for transducers 1, 32 and63 with no incremental delay.

FIG. 21 is a graph of transmit waveforms for transducers 1, 32 and 63with incremental delay.

FIG. 22 is a graph showing variation of focal range with frequency,using a far focus of 140 mm, a near focus of 40 mm and a near focallimit of 28 mm.

FIG. 23 is a graph showing the track of an ultrasound line with a radiusof 500 mm.

FIG. 24 is a graph showing a modified waveform.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following discussion first discusses general system considerations,and then turns to a detailed discussion of individual components of thepreferred system.

System Overview

FIG. 1 is a block diagram of an ultrasonic imaging system whichincorporates a preferred embodiment of this invention. A transmitbeamformer 12 applies analog transmit voltage waveforms via amultichannel switch 14 to an array of transducers 16. The transducers 16each receive a respective transmit waveform and generate a respectiveultrasonic transducer pressure waveform. The ultrasonic transducerwaveforms are timed and shaped as described below to add coherentlyalong a selected spatial axis, with higher frequency components of theultrasonic waveforms focused at shorter ranges (depths), intermediatefrequency components focused at intermediate ranges (depths), and lowerfrequency components focused at longer ranges (depths). By way ofexample, frequency components centered at 7, 5 and 3 MHz can be focusedat ranges of 40, 90 and 140 mm, respectively.

This frequency-dependent focus concentrates higher frequency ultrasonicwaves at shorter ranges where they are most useful. Body attenuationincreases with higher frequencies, which makes higher frequencies lessuseful at long ranges.

Echoes from body structures are detected by the transducers 16, whichgenerate respective receive voltage waveforms. These receive waveformsare applied Via the multichannel switch 14 to a receive beamformer 18,which applies suitable delays and filters to the receive waveforms tocreate a coherent sum for selected points along the spatial axis. Echoesare received sooner from closer ranges, which as explained above areassociated with higher frequency components of the transmittedultrasonic waveforms.

In one mode of operation, the receive beamformer selects delays to focusat progressively longer ranges along the line, thereby sampling multiplepoints along the line. In order to take advantage of the time-varyingfrequency distribution of ultrasonic energy along the line of focus, thereceive beamformer 18 preferably includes a time-varying bandpass filterthat attenuates frequency components of the receive waveforms other thanthose characteristic of the focal range of interest. In the aboveexample, this bandpass filter is centered at 7, 5, and 3 MHz at times2·40/c, 2·80/c, and 2·140/c respectively, where c is the speed of soundin the body. The center frequency of the bandpass filter variesprogressively from 7 MHz at 2·40/c to 3 MHz at 2·140/c.

Transmit Waveform Determination

Transmit waveforms having the frequency-dependent focus characteristicsdescribed above can be determined as follows.

The first step is to select a starting waveform in the time domain. Thisstarting waveform can, for example, be a pulse h(t) having a Gaussianspectrum, as shown in FIG. 2. This pulse h(t) has a fractional bandwidthof 80% at the -6 dB points, i.e., 2·(f_(HI) -f_(LO))/(f_(HI)+f_(LO))=80%, where f_(HI) is the upper frequency at -6 dB with respectto the maximum level and f_(LO) is the lower frequency at -6dB withrespect to the maximum level. In this specification the notation "80% -6dB bandwidth" will be used for such a pulse. A Fourier transform is thenused to convert the waveform h(t) to the frequency domain to form H(f),having amplitude and phase as shown in FIGS. 3 and 4. The startingwaveform can be modified to take into account the amplitude/phaseresponse of the transducer, as well as amplitude/phase errors in theelectronics of the beamformer. For example, if the transducer is assumedto have an 80% -6 dB bandwidth, a starting waveform having a 150% -6 dBbandwidth results in a net 67% -6 dB bandwidth. Corrections forimperfections in electronics such as amplifiers and smoothing filters,and for amplitude and/or phase errors associated with potential dividereffects between amplifier output impedance and transducer impedancepermit less stringent specifications and therefore lower cost parts tobe used.

Each frequency of interest is then assigned to a particular focal rangeby means of a smoothly varying function g such that Z=g(f), where Z isthe focal range for the frequency f. In this example g is selected suchthat 3, 5 and 7 MHz are assigned to focal ranges of 140, 90 and 40 mm,respectively.

The next step is to determine the actual transmit waveform for each ofthe transducers and for the desired line of focus. By way of example,assume Z=40 mm, f=7 MHz, 128 transducers are arranged with a pitch of0.15 mm, c=1.5 mm/μs, and the line of focus is normal to the transducerarray and passes through the center of the transducer array.

In order to calculate the delays for the 7 MHz frequency components ofeach of the 128 transmit waveforms, the distance and time fromtransducer i to the desired focal point are calculated according to thefollowing formulae: ##EQU1## where X_(i) equals the spacing of thei^(th) transducer from the center of the transducer array.

For example, for Z=40 mm, and one of the two transducers closest to thecenter, ##EQU2## For Z=40 mm and the end transducer, X=(63.5×0.15mm)=9.52 mm and ##EQU3## Thus, the 7 MHz component of the end transducermust be advanced by 27.4-26.7=0.7 μs with respect to the 7 MHz componentof the central transducers. `advanced` equals `negative delayed` Thisdelay can be accomplished in the frequency domain by multiplying H(f) bye^(-j2)πft, where t=-0.7 μs and f=7 MHz for this particular frequencycomponent. The process is repeated for all transducers and all frequencycomponents (and associated focal ranges) of interest.

The foregoing example relates to a center scan line that is straight andnormal to the transducer array. A similar approach can be used foroff-center and curved scan lines, as long as X_(i) and Z are selectedproperly. That is, the range calculation should use X_(i) and Z asmeasured from the intended focal point to the i^(th) transducer. Bypositioning the intended focal point properly, delays for angled scanlines and curved scan lines can readily be determined.

In many cases it will be preferable to avoid negative delays (advances)that imply transmit waveforms having non-zero values before t=0. Thiscan be done by calculating the greatest expected end-to-center delaydifference for the entire transducer array. This greatest delaydifference generally occurs at the nearest focal distance, and isassigned as a constant denoted Delay₋₋ Max. The required time advances(negative delays) discussed above may now be added to Delay₋₋ Max todetermine the time value to be used in the frequency domain delayoperations, thereby avoiding all negative delays. Of course, anyconstant value greater than Delay₋₋ Max is also suitable.

This process is repeated for all frequencies for the i^(th) transducerto produce H'_(i) (f), having the amplitude and phase shown in FIGS. 5and 6, respectively. The frequency domain function H'_(i) (f) is thenconverted by use of an inverse Fourier transform to form the time domainfunction h'_(i) (t), which is the transmit waveform for the i^(th)transducer.

FIG. 7 shows the transmit waveforms for five of the transducers h₀, h₃₁,h₆₃, h₉₅, h₁₂₇, where h₀ and h₁₂₇ are the transmit waveforms for the endtransducers, and H₆₃ is the transmit waveform for one of the two centraltransducers. Note that in each case all of the frequency components inany one transmit waveform are combined in a single burst of energy or asingle frequency modulated pulse signal, rather a sequence of multipleunmodulated pulses. Each transmit waveform is a continuously, constantlyvarying signal, rather than multiple pulses separated by a non-varyingperiod lasting more than two times the period of the lowest frequencywithin the -6 dB bandwidth of the transmit waveform. FIG. 8 shows thetransmit waveforms of FIG. 7 filtered with a bandpass filter centered at3 MHz. The dotted line in FIG. 8 shows the curved wavefront of the 3 MHzcomponents, that causes these lower frequency components to focus at thelong range of 140 mm. FIG. 9 shows the transmit waveforms of FIG. 7filtered with a bandpass filter centered at 7 MHz. The dotted line inFIG. 9 shows the more deeply curved wavefront of 7 MHz components, thatcauses these higher frequency components to focus at the short range of40 mm.

Note that the transmit waveform for each transducer includes a widerange of frequency components, and the delays for individual frequencycomponents are selected such that the separate frequency components ofeach transmit waveform are focused at respective focal ranges. Thetransducer waveforms produced by the transducer array as a wholegenerate a continuous line focus rather than a point focus, anddiffering frequency components are focused at differing ranges or depthsalong the line. At least for some of the transmit waveforms, the variousfrequency components are contained in a single burst of energy.

The foregoing discussion illustrates only one approach to determiningthe transmit waveforms. Many modifications and alternatives arepossible, including the following.

The transmit waveforms may be shaped to reduce the effect of ringing inthe waveforms of the transducers at the end of the array by usingconventional aperture apodization techniques to emphasize the responseof the center elements at the expense of the end elements. Low-passfiltering may be used on the transmit waveforms for the end elements tosuppress high frequency ringing, which is largely due to rapidlychanging phase at higher frequencies. Additionally, higher frequencycomponents may be focused at longer ranges for the end transducers thanfor the center transducers. It is often not necessary to focus any partof the pressure wave from end elements at extremely close ranges, and byfocusing all of the pressure wave from end elements at longer ranges,high phase changes and associated ringdown can be reduced.

Additionally, the delay profile can be continued down to 0 Hz and up tobeyond 10 MHz. Beyond the upper band edge of the transducer, it may bedisadvantageous to continue to reduce the focal distance at a constantrate. A minimum focal range can be defined, which higher frequenciesapproach asymptotically. As mentioned above, the near focal limit maynot be the same for the end transducers as for the center transducers.

The transmit waveforms may additionally be designed to compensate forbeamforming distortions. For example, since different frequencycomponents are attenuated by different amounts in the body, lowfrequency components may be enhanced in amplitude to increase the energyfocused at long range targets. To the extent that the effective velocityof ultrasonic waves in the body varies with frequency, such variationscan be taken into account in calculating the delays used in determiningthe transmit waveforms.

It is known in the art that lower frequencies can be used for off-axisultrasound scan lines to reduce the adverse effect of grating lobes dueto undersampling at high frequencies for wide element spacing. Thisapproach can readily be used in determining the transmit waveforms forsuch scan lines.

The previous discussion has related to the objective of producingtemporally compact wave-forms along the scan line. In certainapplications it is desirable to produce temporally long waveforms. Codedwaveforms of the type described by M. O'Donnell in IEEE Trans. UFFC Vol.39, No. 3, pp. 341-351 may also be used with this invention. Thesewaveforms, which are essentially `chirp` waveforms, have the advantageof higher signal to noise since they increase pulse energy withoutincreasing peak power and hence take advantage of the fact thatregulatory limits on peak acoustic power are more burdensome than thelimits on peak acoustic energy in this application. (Signal to noise isrelated more to signal energy than signal power). Since the differentfrequency components are focused to different points, the nature of thefocused waveform will vary with range. Nevertheless, by filtering to areduced bandwidth (e.g., 30% -6 dB fractional bandwidth), the resultantwaveform will contain well focused components. Another feature of`chirp`-like waveforms is that if the low frequencies occur earlier thanthe high frequency components, the total temporal spread in thewaveforms applied to the end elements may be reduced. FIG. 10illustrates the high and low frequency components in such a case. Notethat the total delay from the start to finish of the transmit waveformsis reduced in FIG. 10 as compared to FIG. 11, which shows the alternaterelationship.

In the present invention, a preferred `chirp` waveform may be developedas follows for substitution in place of the starting waveformillustrated in FIG. 2. The design of this `chirp` waveform scales theincremental delay between successive frequency components with theperiod of the particular frequency component. These delays are appliedto the frequency components making up the original pulse, which may,like the pulse illustrated in FIG. 2, have a Gaussian spectral envelopeand linear phase for all frequency components. For simplicity, thecurrent discussion considers a discretely sampled spectrum comprisingfrequency samples at f(j), where f(j) is the frequency of the jthsample. Generally, waveforms in this invention are continuous in boththe time and frequency domains. This condition may be obtained byletting discrete sample intervals tend to infinitesimally small values.##EQU4## n = number of elements in the transducer array, z = near focaldistance for high chosen frequency component (40 mm in this example),

N = number of frequency samples between low frequency component f_(LO)(3 MHz in this example) and high frequency component f_(HI) (7 MHz inthis example)

f_(c) = center frequency=5 MHz in this example,

f.sub.(j) =frequency of j^(th) sample.

Delays are calculated over the range 0.3·f_(LO) to 2·f_(HI), wheref_(LO) =3 MHz and f_(HI) =7 MHz in this example. Delays for sampleswhere f(j) is less than 0.3·f_(LO) are set equal to zero. Delays forsamples where f(j) is greater than 2·f_(HI). are set equal to the delayfor the sample corresponding to f(j)=2·f_(HI). A chirp waveform has theadvantage that energy is spread out in time and hence peak power islowered, reducing the risk of exceeding government regulated powerlevels.

The waveforms corresponding to a number of separate ultrasound lines canbe calculated and then summed prior to application to the transducerssuch that ultrasonic energy is focused along several different scanlines. The transmit scan lines may be straight or curved, as desired. Ifthe transmit scan lines are curved, the azimuthal position of the focusvaries with range and frequency. The receive beamformer would preferablyaccommodate this change, and the scan converter would write to X-Ylocations using curved rat her than straight line acoustic data. As anexample, in the scan converter described by S. C. Leavitt et al. `A ScanConversion Algorithm for Displaying Ultrasound Images` (Hewlett-PackardJournal, October 1983, pp. 30-34), the X-Y Raster State Machine (page33) could be programmed with a sequence of X-Y pixel addresses followinga curved rather than straight trajectory. Also, each transmit scan linemay be spread out in width. For example, in some applications it may bepreferable to spread or defocus the beam to a width such as 4° to allowmultiple receive scan lines for a single transmit scan line.

Transmit Beamformer

Once the desired transmit waveforms have been determined as discussedabove, the transmit beamformer 12 can be implemented as shown in FIG. 12to generate the previously determined transmit waveforms.

The transmit beamformer 12 includes N channels, one for each of thetransducers 16 (FIG. 1). Each channel includes a delay memory 20, awaveform memory 22, and a delay counter 24 (FIG. 12). The delay memory20 includes 256 words 26, one for each possible steering angle orultrasound scan line. The waveform memory 22 includes 256 sections 28,one for each possible steering angle. Each word 26 is set equal to anegative number equal to the number of clock cycles that elapse betweena start of line signal and the first non-zero value of the associatedwaveform. For simplicity, it is assumed that zero is defined as the mostsignificant bit equal to 1 and all other bits equal to 0. Hence, themost significant bit becomes an enable signal for the memory. Eachsection 28 stores a respective waveform, for example as 64 or 128successive eight bit words. When a section 28 is read with a 40 MHzclock, the resulting sequence of digital values defines a waveformapproximately 1.6 to 3.2 μs in duration. The delay memory 20 is notrequired, but it reduces memory requirements for the waveform memory 22.This is because the delay memory 20 eliminates the need to store a largenumber of leading zeros when the ultrasound line is steered at a largeangle.

In use, each channel responds to a scan line selection signal on line 30by loading the word 26 for the selected scan line into the delay counter24, and by enabling the selected section 28 of the waveform memory 22.Typically, each word 26 stores a negative binary integer equal to thedesired delay before the first non-zero value of the respectivewaveform.

The delay counter 24 responds to a start of scan line signal on line 32by incrementing the stored value with each cycle of a 40 MHz clock. Whenthe counter 24 increments to zero, it enables the waveform memory 22.Subsequently generated values of the counter 24 (incrementing now fromzero upwards) become address values for the memory 22. As each word ofthe section 28 for the selected scan line is addressed, thecorresponding eight bit word is read and applied to a digital-to-analogconverter 34. The analog output signal of the converter 34 is passedthrough a low-pass filter such as a Bessel filter 36 to reduce samplingeffects and then to an amplifier 38. The output of the amplifier 38 isthe transmit waveform discussed above that is applied to the respectivetransducer 16 via the multichannel switch 14 (FIG. 1).

In general, there is considerable similarity between waveforms appliedto adjacent transducers 16 and between waveforms of adjacent lines. Anumber of approximations can be used which take advantage of theredundancy in the information stored in the waveform memory 22 to reducememory requirements.

Another approach is shown in FIG. 13, which includes many of the samecomponents as those discussed above in conjunction with FIG. 12. Thecentral difference between the systems of FIGS. 12 and 13 is that eachtransducer channel of the system of FIG. 13 uses only a single waveformmemory section 28 that stores only a single waveform made up of 64 or128 eight bit words. The waveform stored in the waveform memory section28 may be the waveform calculated for the center scan line. The systemof FIG. 13 functions as described above in conjunction with FIG. 12,except that the scan line number select signal does not select one ofmultiple waveform memory sections. All of the waveforms for all of theultrasound scan lines are identical in shape. They differ from oneanother only in that linear delays are applied to successive scan linesto effect scan line steering.

When the approach of FIG. 13 is used it should be understood that aseach scan line is steered farther from the central scan lineperpendicular to the transducer array, an error in the focusingcomponent causes the focal points to approach the transducers by afactor of (cosθ)², where θ is the steering angle measured with respectto the perpendicular. This focusing error results from the fact that theeffective pitch between adjacent transducers is modified by cosθ fornon-perpendicular steering angles, and the resulting delay is modifiedby (cosθ)².

As an improvement to compensate partially for this effect, one cancalculate the delays required to focus at the desired range (for example140 mm). One can then calculate the delays required to focus at 70 mm(i.e. the range to which the beam is actually focused if it wasoriginally focused at 140 mm but has been steered to 45°) The differencein the delays for 140 mm and 70 mm can be applied to the waveformsdiscussed above to compensate for this focusing error. This correctionapplies exactly only to one frequency component, and other frequencycomponents (and associated other ranges) will not be exactly corrected.

FIG. 14 shows another system that uses an interpolator to reduce memoryrequirements as compared to the system of FIG. 12. In the system of FIG.14 the delay memory 20, the delay counter 24 and the components 34, 36and 38 are as described above. In this case the waveform memory 22includes sections 28 that store only every fourth (or other power oftwo) waveform for the respective transducer. The actual waveform used byintermediate lines is interpolated digitally using the shifters 40, 42,the summer 44 and the actual waveform memory 46.

A central controller provides first and second waveform select signalswhich select the two waveforms to be used for the interpolation. Thiscontroller also generates shifter control signals. The first waveformidentified by the first waveform select signal is applied to the firstshifter 40, and the second waveform identified by the second waveformselect signal is applied to the second shifter 42. Each of the shifters40, 42 supplies outputs equal to selected ones of the following: thecorresponding waveform divided by 1, the corresponding waveform dividedby two, and the corresponding waveform divided by four. The outputs ofthe shifters 40, 42 can be obtained at high speed by simple shiftingoperations. The summer 44 sums the various signals generated by theshifters 40, 42 to generate the actual waveform, which is stored in theactual waveform memory 46.

This actual waveform memory 46 stores 128 eight bit signals. The delaycounter 24 is loaded with the appropriate delay from the delay memory20, and then clocked beginning at the start-of-scan-line signal. Whenthe value in the delay counter 24 goes positive, it addressesconsecutive words in the actual waveform memory 46 and applies them tothe digital-to-analog converter 34.

Table 1 provides further information regarding the operation of thewaveform memory 22 and the shifters 40, 42.

                  TABLE 1                                                         ______________________________________                                        Line First Waveform                                                                           Second Waveform                                                                            Shifter 40                                                                            Shifter 42                               No.  Select Signal                                                                            Select Signal                                                                              ÷1                                                                            ÷2                                                                            ÷4                                                                            ÷1                                                                            ÷2                                                                            ÷4                       ______________________________________                                        0    0          4            1   0   0   0   0   0                            1    0          4            0   1   1   0   0   1                            2    0          4            0   1   0   0   1   0                            3    0          4            0   0   1   0   1   1                            4    4          8            1   0   0   0   0   0                            5    4          8            0   1   1   0   0   1                            ______________________________________                                    

As shown in Table 1, for scan lines 0-3 the scan line 0 and 4 waveformsare applied to the shifters 40, 42 respectively. Scan line 0 is equal tothe waveform stored in scan line 0 of the waveform memory 28, becauseonly the ÷1 output of the shifter 40 is enabled. Similarly, scan line 1is equal to the sum of 1/2 plus 1/4 of the waveform for scan line 0 plus1/4 of the waveform for scan line 4. The waveform for scan line 2 isequal to 1/2 form for scan line 0 plus 1/2 of the waveform for scan line4.

Since the delay between adjacent scan lines is at least 50 microsecondsfor a target at a distance of 40 mm, there is sufficient time toaccomplish simple digital interpolations using shifts, which can behardwired, and selected adds of shifted components from adjacent lines.

Interpolated waveforms can also be determined by generating two digitalwaveforms, converting them to their analog counterparts, and thenperforming the desired interpolation as a weighted analog sum usingcontrolled gain amplifiers and a summer. This approach is an analogversion of the system of FIG. 14. It is possible to use interpolationtechniques similar to those of FIG. 14 to interpolate between successivetransducers. Also, it is possible to store only a limited set ofwaveforms (either for a limited set of lines and/or a limited set oftransducers). One would use the closest stored waveform for transducersand/or lines which are not explicitly stored. The extent of hardwaresimplicity afforded by this technique is balanced with a slight loss ofperformance.

FIG. 15 relates to another system which takes advantage of the fact thatthere is considerable redundancy between the waveforms applied toadjacent scan lines for any given transducer. As shown in FIG. 15, thewaveform memory 48 stores the complete waveform for a given transducer,such as the waveform for scan line 0. A value ΔWFM is stored for eachsubsequent scan line. The summer 50 is initially loaded with thewaveform for scan line 0, and ΔWFM1, ΔWFM2, . . . ΔWFMN are thensuccessively added. In each case ΔWFMn is the increment between thewaveform for scan line (n-1) and the waveform for scan line n. Thecontents of the summer 50 represent the actual waveform for thetransducer of interest and the corresponding scan line. This actualwaveform is clocked by the delay counter 24 into a digital-to-analogconverter 34. For example, the summer 50 can add the waveform for scanline 1 to the value of ΔWFM for scan line 2 to generate the actualwaveform for the second scan line and the respective transducer. Theapproach of FIG. 15 is especially well suited for use in systems inwhich scan lines are fired in consecutive order. The basic approachillustrated in FIG. 15 can be adapted for successive transducers insteadof or in addition to successive scan lines.

Arbitrary waveforms of the type described above can be generated withconventional function generators, such as the Model DS345 synthesizedfunction generator of Stanford Research Systems. An array of suchdevices is a practical approach to implementing the transmit beamformer12 in the shortest amount of time, particularly when a smaller number oftransducers such as sixteen is used.

Since the capacity of the GPIB which connects a computer to severalDS345's is limited, it may be necessary to use more than one computerand build the beamformer with subarrays with separate computers andGPIB's. This system is still practical since once all the DS345's in thedifferent subarrays have been programmed they can be triggered from asingle external synchronizing source.

There are other means for generating approximations to the waveformsdiscussed above. One approach is to produce a square wave burst with aperiod between successive transitions that determines the fundamentalfrequency. A low-pass filter can be applied to remove the harmonics andto smooth the waveform to make it more like one of the waveformsdiscussed above. This technique would also achieve the effect offocusing various frequency components at various respective ranges.

It is anticipated that the programmable waveform transmit beamformerdescribed in Cole et al. U.S. patent application Ser. No. 08/286,652,filed Aug. 5, 1994, and assigned to the assignee of the presentinvention can be adapted for use with this invention.

Transducers

A wide variety of transducers 16 can be used, and this invention is notlimited to the linear transducer array discussed above. The techniquesdiscussed above of delaying separate frequency components (so as toachieve a multiple focal ranges) may be applied to two dimensionalarrays having M azimuth elements and N elevation elements, or to a 1.5dimension array which will typically have a small number of elements inthe elevation direction, such as 3, 5 or 7.

A plano-concave transducer array can be used in which differentfrequency components are focused at different ranges in elevation. Seefor example the discussion in the continuation in part of Hanafy U.S.patent application Ser. Nos. 08/117,869 now U.S. Pat. No. 5,938,998 and08/117,868 now U.S. Pat. No. 5,387,674, filed Sep. 7, 1993.

The Receive Beamformer

The receive beamformer 18 preferably includes a dynamic receive focusingsystem that allows the focus of the receive beamformer to be changed ata high rate in order to follow as accurately as possible the range alongthe ultrasonic scan line corresponding to the currently arrivingsignals.

Preferably, the receive beamformer 18 includes a time-varying adjustablebandpass filter which is adjusted in real time to emphasize thefrequency of the currently arriving signals. Green U.S. Pat. No.4,016,750 describes a simple analog implementation far such atime-varying filter. A high-pass filter can be substituted for abandpass filter. The body acts as a low-pass filter, and for this reasona high-pass filter may be sufficient to achieve the desired effect.

When a time-varying bandpass filter is used, it can slide from above 7MHz to below 3 MHz if desired. The slide rate function need not beuniform with respect to time. The optimum bandwidth and filtercharacteristics of the sliding filter can best be determined fromexperience and by using design tools. A narrow bandwidth will givehigher focusing accuracy but relatively poor axial (range) resolutiondue to ring down. The frequency downshift related to natural bodyattenuation should be taken into account in the design of such a filter.

Heterodyne time-varying filters may also be used in the receivebeamformer 18. Analog ultrasound systems frequently use a heterodynetechnique to shift radio frequency pulses generated by the transducerdown to an intermediate frequency, e.g. 1-3 MHz. See for example MaslakU.S. Pat. No. 4,140,022, and Pummer U.S. Pat. No. 5,218,869. If a narrowbandpass filter is employed on an intermediate frequency signal of 2MHz, a time-varying bandpass filter will be formed which only passescomponents corresponding to the original components of 7 MHz down to 3MHz as the local oscillator is varied from 9 MHz to 5 MHz. Atime-varying local oscillator may be realized by using a voltagecontrolled oscillator circuit, where the voltage determining the desiredlocal oscillator frequency is derived via a digital-to-analog converterfrom a value supplied by the system computer controller.

A time-varying, sinusoidal-frequency waveform may also be generatedusing any one of a number of digital synthesizer techniques. See W. F.Egan, "Frequency Synthesis by Phase Lock", Krieger, 1990.

Digital filtering can also be used in the receive beamformer. Adigitized signal may be shifted using quadrature sampling and sampledecimation. Fine shifts in frequency are achieved by means of complexmultiplication with an appropriate complex exponential exp(j*2*π*t*f₀),where f₀ is the amount of shift in frequency. The amount of frequencydownshifting can be varied as a function of time and therefore range.Varying the degree of frequency shifting of the signal with respect to afixed-frequency bandpass filter results in a time-varying portion of theoriginal signal spectrum being passed. A fixed finite impulse response(FIR) bandpass or low-pass filter is applied to the data to yield a netresponse equivalent to a time-varying filter.

It is anticipated that the receive beamformer described in Wright, etal. U.S. patent application Ser. No. 08/286,658, filed Aug. 5, 1994 nowabandoned and assigned to the assignee of the present invention can beadapted for use with this invention.

Other Applications

It should be understood that the applications discussed above have beenprovided only by way of example. The present invention can be adapted toa wide range of applications, and is not to be limited to the specificapplications discussed in this specification.

For example, the present invention is well suited for use in multiplebeam systems, as well as in systems that downshift frequency foroff-axis scan lines. Wright, et al. U.S. patent application Ser. No.08/286,524, filed Aug. 5, 1994, and assigned to the assignee of thepresent invention, describes such systems in detail.

This invention may also be used in conjunction with non-linear contrastagents, as described by B. Schrope, et al. ("Simulated Capillary BloodFlow Measurement Using a Nonlinear Ultrasonic Contrast Agent,"Ultrasonic Imaging, 14, 134-152 (1992)). These agents possess a resonantfrequency and, when subjected to high pressure intensity at thisfrequency, will cause acoustic pressure waves to be emitted at thesecond, or higher, harmonic of the fundamental transmitted frequency. Inthe receive signal path, echoes at the fundamental frequency arefiltered out to produce an image of only the contrast agents--whichtypically follows closely the flow of blood through the medium ofinterest. In transmission, the bandwidth of the emitted signal iscontrolled so that practically no second harmonic energy is transmitted,which would result in echoes being received which would beindistinguishable from the desired second harmonic contrast agentinduced signals. The present invention is of particular importance inthis application, since it permits a high acoustic pressure to bemaintained over a greater depth of field than in a fixed focus system.Maintaining the acoustic pressure at high safe levels is preferred,since at these pressures the second harmonic non-linear generation ismost effective. In the present invention one might transmit 3 MHz energyto a deep focus and 3.5 MHz energy to a more shallow focus. In receive,a time-varying filter would first detect signals at 7 MHz and then varydownwards in frequency to detect signals at 6 MHz.

Further Best Mode Details

As described above, a wide variety of waveforms can be used, dependingupon the particular application. The following discussion focuses on onepreferred embodiment, and is not intended to be limiting.

For this example the transducer is assumed to be a 5 MHz, 128 element,0.15 millimeter pitch transducer. The original transmit pulse is a 150%bandwidth Gaussian pulse, and three sections are filtered out at 3, 5, 7MHz using a Butterworth filter. A Chebyshev, Bessel or digital finiteimpulse response (FIR) filter may be a suitable alternative. Theresulting spectra of the three-filtered sections are shown in FIG. 16.The filter was chosen to have approximately 30% bandwidth (with respectto the center frequency) at the -6 dB points. The bandwidth and numberof filter poles are constant. Generally, it is assumed that 3 MHz energyis focused at 140 mm, 5 MHz energy at 90 mm, and 7 MHz energy at 40 mm.

FIG. 17 shows the variations in focus as a function of frequency andrange. At 3 MHz, the entire transducer array is focused at 140millimeters. At 7 MHz only the center of the array is focused at 40millimeters. The near focal point at 10 MHz is 28 millimeters.

Low-pass filtering was applied to all elements, with more filtering ofthe end elements as suggested above. The cutoff frequencies across thearray of transducer elements varied from 9 MHz at the center of thearray to 5 MHz at the end of the array. The element-to-element functionfor determining the cutoff was linear. FIG. 18 illustrates the spectraof the elements extending from the center to the end of the transducerarray. Half circle amplitude apodization has also been applied. Theminimum amplitude level at the ends is 0.2, though it could be loweredto further suppress side lobes. FIG. 19 is a contour plot illustratingthe data of FIG. 18.

FIG. 20 illustrates waveforms to be applied to the end transducer, atransducer midway between the end and the center, and the centertransducer. A very substantial reduction in ringdown is readily evident.The duration of the pulse is limited more by the near focal zone delayprofile than by ringdown.

FIG. 21 illustrates the result when incremental delays are applied tosuccessive frequency components. In this case the delay is 0.27microseconds between 3 MHz and 7 MHz. The transfer of high frequencyenergy toward the temporal waveform center of the central element pulseis evident, as shown by the asterisk. The total duration of the pulsesis approximately 1.5 microseconds.

FIG. 22 illustrates a focal range versus frequency plot. Over the usefulrange (3-7 MHz) the focal range changes linearly. The receive bandpassfilter ideally starts at a center frequency of 7 MHz and continues at 7MHz until t=2·40 mm/c. Thereafter, the center frequency decreases withrange linearly until it reaches 3 MHz at t=2·140 mm/c. At that point thedownward ramp stops and remains at 3 MHz. Level regions at the beginningand end of the scan line minimize the loss of useful signal. The slopemay be modified in practice to accommodate the frequency downshiftversus time due to body attenuation. Although the variation of focalrange with respect to frequency is linear over the primary operatingrange in FIG. 22, other functions describing the relationship betweenfocal range and frequency, which may result in better overallperformance, may be derived by analysis or experimentation. As anexample, since focusing delays are approximately related to the inverseof the focal range, it may be preferable to make focal range an inversefunction of frequency so that range change is greatest where theresulting delay changes are smallest.

FIG. 23 illustrates a graph of an ultrasonic scan line that is focusedso that after bandpass filtering it follows an arc of radius 500 mm.

In general, the maximum intensity should be at the deeper focal zones,where the signal-to-noise ratio is the lowest. This can be accomplishedby skewing the Gaussian spectrum described above by low-pass filteringit to emphasize low frequency energy at the expense of 5 and 7 MHzenergy. A suitable filter is a 4 pole, 3.5 MHz Butterworth low-passfilter. When such a filtered Gaussian was used, the maximum intensitywas achieved at 140 mm.

More complicated filter approaches can be used. For example, in asituation where the peak intensity is at the 5 MHz focus, one mayreplace the pure Gaussian spectrum discussed above with one having aflat top and a Gaussian-like roll-off. See FIG. 24. In this way goodperformance can be maintained at both 3 MHz and 7 MHz.

The attached appendix provides a listing of digitized sample values of16 transmit waveforms suitable for use for transducer elements 0, 4, 8 .. . 60 of the linear array described above, focused perpendicularly tothe transducer array. The waveforms for intermediate transducer elementsmay be found using the interpolation scheme described above. Also, thewaveforms for elements 67 to 127 may be found from the mirror image ofthe data shown for elements 0 to 60, i.e., Channel 127=Channel 0,Channel 123=Channel 4, etc. Channels 61-66 may use the values forChannel 60 since the variation among center channels is negligible. Seethe difference between Channels 60 and 56. In the appendix, row A liststhe transducer numbers 0, 4, 8 . . . 3C(Hex), rows B and C list thedelay values for each respective waveform in hex, and rows 00-3F list 64successive values for each waveform, when read as a column. The delayvalues of rows B and C assume 200(Hex) is zero, and 0-1FF(Hex) areconsecutive negative integers. The waveform values of rows 00-3F arelinear, with 80(Hex) equal to zero, and 81-FF(Hex) equal to positiveintegers and 0-7F(Hex) equal to negative integers. The appendix assumesa clock rate of 40 MHz. In this case 64 memory samples are justsufficient. In a commercial design, considerable flexibility is offeredwhen the memory size is increased to 128 samples.

Conclusion

The systems described above provide a number of important advantages.Since the transmit beamformers provide a line focus rather than a pointfocus, there is a reduced requirement for user fine tuning. This canreduce or eliminate the need for a user to select the correct focaldepth or to resort to multi-zone imaging. These systems can give highlyadvantageous resolution at high frame rates without resorting tomulti-zone techniques. By eliminating or reducing the need formulti-zone techniques, frame rates are increased and image artifactproblems associated with the need to combine images are reduced. Thereis the potential for increased net transmitted power without exceedingpeak intensity limits in view of the use of a line focus.

Of course, it should be understood that many changes and modificationscan be made to the preferred embodiments described above. This inventionis not limited to use with ultrasonic beamformers, but can also beadapted for use in sonar, radar, and other applications. It is thereforeintended that the foregoing detailed description be regarded asillustrative rather than limiting, and that it be understood that it isthe following claims, including all equivalents, that are intended todefine the scope of this invention.

                                      APPENDIX                                    __________________________________________________________________________    Row A 00 04 08 0C 10 14 18 1C 20 24 28 2C 30 34 38 3C                         B     1E6   1E2   1DE   1DA   1D6   1D5   1D5   1D5                           C        1E4   1E0   IDC   1D8   1D6   1D5   1D5   1D5                        Row 00                                                                              82 81 80 80 7F 7E 7D 7D 7C 7C 7C 7C 7C 7C 7C 7C                             01                                                                              7F 7E 7E 7E 7E 7D 7D 7D 7C 7C 7C 7C 7C 7C 7C 7C                             02                                                                              7C 7C 7C 7D 7D 7D 7D 7C 7C 7C 7B 7B 7B 7B 7B 7B                             03                                                                              7D 7D 7E 7E 7E 7E 7D 7C 7B 7B 7B 7B 7B 7B 7B 7B                             04                                                                              82 82 81 80 7F 7E 7D 7B 7B 7B 7B 7B 7B 7B 7B 7A                             05                                                                              84 83 82 80 7E 7D 7C 7B 7A 7A 7A 7A 7A 7A 7A 7A                             06                                                                              7F 7F 7E 7D 7C 7B 7B 7A 7A 7A 7A 7A 7A 7A 7A 7A                             07                                                                              79 79 79 79 7A 7A 7B 7B 7B 7A 7A 7A 7A 7A 7A 7A                             08                                                                              7A 79 79 7A 7B 7B 7B 7B 7B 7B 7B 7B 7A 7A 7A 7A                             09                                                                              80 80 80 7F 7E 7D 7C 7B 7B 7B 7B 7B 7B 7B 7B 7B                             0A                                                                              85 85 84 81 7F 7D 7B 7B 7B 7B 7C 7B 7B 7B 7B 7B                             0B                                                                              81 80 7F 7E 7C 7B 7A 7A 7C 7C 7C 7C 7C 7C 7C 7C                             0C                                                                              78 76 76 76 77 78 79 7B 7D 7C 7D 7D 7D 7D 7C 7C                             0D                                                                              76 74 74 75 77 79 7B 7D 7E 7D 7E 7E 7E 7D 7D 7D                             0E                                                                              7D 7D 7C 7C 7D 7D 7E 7F 7F 7F 7F 7F 7F 7F 7F 7F                             0F                                                                              83 85 86 84 82 80 7F 7F 80 80 81 80 80 80 80 80                             10                                                                              83 83 83 82 80 7E 7D 7E 80 81 82 82 82 81 81 81                             11                                                                              7C 78 77 77 78 79 7B 7E 82 82 83 83 83 83 83 83                             12                                                                              76 72 70 71 74 78 7D 81 85 84 85 85 85 85 85 85                             13                                                                              77 78 78 79 7C 7F 83 86 88 86 87 87 87 86 86 86                             14                                                                              7E 82 86 88 88 88 88 88 88 88 89 89 89 88 88 88                             15                                                                              84 86 89 8B 8A 88 86 86 88 8A 8B 8B 8B 8A 8A 8A                             16                                                                              83 82 80 7F 7F 7F 81 84 89 8B 8D 8D 8D 8C 8C 8C                             17                                                                              7D 79 75 73 74 79 7F 87 8D 8C 8E 8E 8E 8E 8E 8E                             18                                                                              77 75 75 77 7A 80 88 8E 92 8F 90 90 90 90 90 90                             19                                                                              78 7A 80 87 8D 91 94 95 94 92 92 92 91 92 92 92                             1A                                                                              80 85 8C 92 97 98 96 92 90 93 93 93 93 93 93 93                             1B                                                                              88 8C 8E 8E 8E 8C 8B 8A 8C 92 92 93 93 93 93 93                             1C                                                                              89 89 86 81 7D 7D 81 88 8E 8F 90 92 92 92 93 93                             1D                                                                              82 7F 7C 7A 7A 7F 88 91 94 8E 8D 8F 90 90 91 91                             1E                                                                              7A 78 79 80 8A 93 9B 9C 94 8F 8A 8B 8C 8D 8D 8D                             1F                                                                              79 7D 84 90 9C A4 A4 9A 88 8D 85 85 86 87 87 87                             20                                                                              82 8B 95 9C A0 9E 94 85 74 83 7B 7D 7E 7E 7F 7F                             21                                                                              8E 97 9C 9B 93 87 7A 70 67 72 6C 71 73 74 74 74                             22                                                                              94 97 94 8C 82 77 71 6E 68 63 5D 62 66 67 68 68                             23                                                                              8E 8B 83 7C 7A 7C 7F 7B 6A 5C 53 55 58 5A 5B 5B                             24                                                                              82 7C 79 7A 83 8D 8D 7A 5A 5A 50 4D 4E 4F 50 4F                             25                                                                              79 79 7F 8A 94 93 80 5B 3D 53 50 4D 4B 4A 4A 4A                             26                                                                              7C 85 92 9D 9A 81 57 32 35 49 56 56 52 50 4F 4E                             27                                                                              8A 98 A2 9E 86 5B 30 29 5F 4E 6C 6B 67 63 61 61                             28                                                                              9A A3 9E 85 5C 34 29 54 AE 78 9A 8E 89 84 82 82                             29                                                                              9F 9A 82 5B 34 27 4B 9E EF C0 D5 BE B4 AF AC AC                             2A                                                                              92 7C 59 37 29 44 8A DB F5 F9 F9 E7 DB D5 D3 D3                             2B                                                                              77 59 3A 2E 45 80 C8 EC C1 F4 E1 EC E9 E6 E5 E6                             2C                                                                              SC 41 36 48 7B BC E5 CE 79 AF 90 BA CB D0 D3 D4                             2D                                                                              4D 42 50 79 B1 DA D3 94 46 59 39 63 83 93 99 9A                             2E                                                                              51 5A 7B A9 CE CE A0 5C 3A 2B 17 1C 33 45 4D 4D                             2F                                                                              66 7E A2 C2 C7 A6 6C 41 4E 35 39 16 0E 13 16 16                             30                                                                              81 9D B7 BE A7 79 4F 4B 73 5F 78 50 2E 1E 19 18                             31                                                                              96 AB B3 A4 81 5B 5D 6A 91 87 A2 99 7C 63 57 57                             32                                                                              A0 A8 9F 85 66 57 64 86 9A 9B A5 B8 BA AF A6 A7                             33                                                                              9D 99 87 6E 5F 64 7C 92 90 9B 93 A6 BE CB CE CE                             34                                                                              92 86 74 66 67 77 8B 8F 80 8C 80 82 94 A8 B4 B4                             35                                                                              85 78 6D 6B 74 84 8D 85 78 7B 74 6F 6B 72 79 79                             36                                                                              7B 72 6F 74 80 88 86 7C 77 73 72 70 64 59 56 55                             37                                                                              77 73 75 7D 85 86 7F 79 7B 76 78 78 74 69 61 61                             38                                                                              77 77 7C 82 84 81 7C 7B 7F 7E 80 7E 83 83 80 80                             39                                                                              79 7C 80 83 82 7E 7C 7E 81 82 84 83 85 8B 8F 8F                             3A                                                                              7C 7F 81 82 7F 7D 7D 80 80 82 82 84 82 84 88 88                             3B                                                                              7F 80 81 80 7E 7D 7E 80 7F 80 7F 81 80 7E 7D 7D                             3C                                                                              80 80 80 7E 7D 7E 7F 7F 7F 7F 7E 7E 7F 7E 7B 7B                             3D                                                                              80 7F 7F 7F 7E 7F 7F 7F 80 7F 7F 7E 7E 7F 7F 7F                             3E                                                                              7F 7F 7E 7E 7F 7F 80 80 80 80 80 7F 7F 80 81 81                             3F                                                                              7F 7E 7E 7E 7F 50 80 80 8C 80 81 81 80 80 80 80                         __________________________________________________________________________

I claim:
 1. An ultrasonic imaging system comprising:a two dimensionaltransducer array; a transmit beamformer operatively coupled with the twodimensional transducer array and operative to generate waveforms atfundamental frequencies; and a receive beamformer operatively coupledwith the two dimensional transducer array and having an adjustablefilter operative to receive echo signals from the two dimensionaltransducer array and to filter out echoes at the fundamental frequenciesand pass echoes at a harmonic frequency.
 2. The ultrasonic imagingsystem of claim 1 wherein the transmit beamformer is operative to focusa first frequency component at a first depth and a second frequencycomponent at a second depth.
 3. The ultrasonic imaging system of claim 1wherein the adjustable filter comprises a time varying filter.
 4. Theultrasonic imaging system of claim 1 wherein the adjustable filtercomprises a digital finite impulse response filter.
 5. The ultrasonicimaging system of claim 1 wherein the adjustable filter comprises afilter operable to provide adjustable decimation.
 6. The ultrasonicimaging system of claim 1 wherein the adjustable filter receives theecho signals and the echo signals are responsive to contrast agents. 7.The ultrasonic imaging system of claim 1 wherein the adjustable filtercomprises a bandpass filter.
 8. The ultrasonic imaging system of claim 1wherein the transmit beamformer is operative to generate waveformscomprising a Gaussian spectrum.